Voltage variable resistor

ABSTRACT

A VOLATGE VARIABLE RESISTOR COMPRISES A SINTERED BODY, ELECTRODES ON OPPOSITE MOJOR SURFACES OF THE BODY AND LEADS CONNECTED TO THE ELECTRODES. THE SINTERED BODY COMPRISES ZINC OXIDE, AND AS AN ADDITIVE, 0.05 TO 15.0 MOLE PERCENT OF AT LEAST ONE MEMBER SELECTED FROMT EH GROUP CONSISTING OF MANGANESE FLUORIDE, MAGNESIUM FLUORIDE, CALCIUM FLUORIDE, CADMIUM FLUORIDE, POTASSIUM FLUORIDE, CHROMIUM FLUORIDE, SODIUM FLUORDIE, COBALT FLUORIDE, COPPER FLUORIDE, FERRIC FLUORIDE, LANTHANUM FLUORIDE AND LITHIUM FLUORIDE.

Feb. 15, 1972 TAKESHI MASUYAMA ET AL VOL'I'AGE VARIABLE RESISTOR Filed April 17, 1970 FIGJ INVENTORS A %m .v. mw m M D W MD! mMom s I EOHH KNSS A o T Y 2 m F afrim EM ATTORNEYS ;United States Patent O 3,642,664 VOLTAGE VARIABLE RESISTOR Takeshi Masuyama, Takatsuki-shi, Mikio Matsuura, Neyagawa-shi, Yoslio Iida, Suita-shi, and Toslioki Amemiya, Tokyo-to, Japan, assigno's to Matsushita Electric Industries Co., Ltd., Kadoma, Osaka, Japan Filed Apr. 17, 1970, Ser. No. 29,415 Claims priority, application Japan, May 2, 1969, 44/34,928 Int. Cl. H01b 1 06 U.S. Cl. 252-519 6 Claims ABSTRACT OF THE DISCLOSURE A voltage varable resistor comprises a sintered body, electrodes on opposite major surfaces of the body and leads connected to the electrodes. The sintered body comprises zinc oxide, and as an additive, 0.05 to 15.0 mole percent of at least one member selected from the group consisting of manganese fluoride, magnesium fluoride, calcium fluoride, cadmium fluoride, potassium fluoride, chromium fluoride, sodium fluoride, cobalt fluoride, copper fluoride, ferrc fluoride, lanthanum fiuoride and lithium fluoride.

This invention relates to compositions of voltage variable resistor ceramics having non-ohmic resistance, and more particularly to compositions of resistors comprising zinc oxide having non-ohmic resistance due to the bulk thereof.

Varions voltage varable resistors such as silicon carbide varistors, selenium rectifiers and germanium or silicon p-n junction diodes have been widely used for stabilization of voltage of current of electrical circuits. The electrical characteristics of such a voltage variable resistor are expressed by the relation:

where V is the voltage across the resistor, I is the current flowing through the resistor, C is a constant corresponding to the voltage at a given current and exponent n is a numercal value greater than 1. The value of n is calculated by the following equation:

log

where V and Vz are the voltages at given current 1 and 1 respectively. The desired value of C depends upon the kind of application to which the resistor is to be put. It is ordinarily desirable that the value of n -be as large as possible since this exponent determines the extent to which the resistors depart from ohmic characteristics.

In conventional varistors comprisin-g germanium or silicon p-n junction diodes, it is diicult to control the C-value over a wide range because the voltage variable property of these varistors is not attributed to the bulk but to the p-n junction. On the other hand, silicon carbide 'varistors have voltage variable properties due to the contacts among the individual grains of silicon carbide bonded together by a ceramic binding material, and the C-value is controlled by changing a dimension in a direction in which the current flows through the varistors. Silicon carbide varistors, however, have a relatively low ice n-value and prepared by firing in non-oxdizing atmosphere, especially for the purpose of obtaining a low C- value.

An object of the present nvention is to provide a composition of a voltage variable resistor having non-ohmic resistance due to the bulk thereof and having a controllable C-value.

Another object of the present invention is to provide a composition of a voltage variable resistor characterized by a high n-value.

A further object of the present invention is to provide a composition of a voltage variable resistor having socalled negative resistance.

These and other objects of the invention will become apparent upon consideration of the following description taken together with the accompanying drawings in which FIG. 1 is a partly cross-sectional view of a voltage variable resistor according to the invention and FIG. 2 is typical V-I characteristics of resistors according to the nvention.

Before proceeding with a detailed description of the voltage varia'ble resistors contemplated by the invention, their Construction will be described with reference to FIG. l wheren reference character 10 designates, as a whole, a voltage variable resistor comprisng, as its active element, a sintered body having a pair of electrodes 2 and 3 applied to opposite surfaces thereof. Said sintered body 1 is prepared in a manner hereinafter set forth and is in any form such as circular, square or rectangular plate form. Wire leads 5 and 6 are attached conductively to the electrodes 2 and 3, respectively, by a connection means 4 such as solder or the like.

A voltage variable resistor according to the inventon has a non-ohmic resistance shown by a curve OP of FIG. 2. The V l characterstics of present voltage variable resistor are further divided into two classes 1) a curve OPS and (2) a curve OPQR. The electrical behavior at a PQ region, is a so-called negative resistance. The PQ region of voltage varia-ble resistor in the second class can be used as a negative resistance device. It is also possible to use the OP region of voltage variable resistor in the second class as a usual varistor which has no negative resistance. A choice of either the first class or the second class is dependent upon the amount of additives as explained hereinafter. The voltage nonlinear property at the OP region can be expressed in terms of C and of Equation l. The negative resistance properties shown at the PQR region are expressed by the negative resistance factor, which is expressed by the following equation:

V I Q 0` 1010 Q/log P where Vp and I are the voltage and current at the point P and V and I are the voltage and current at the point Q. It is desirable that the value of be as large as possible since this factor determines the eXtent of the sharpness of the OPQ curve.

A voltage-variable resistor according to the invention comprises a sintered body of a batch composition consisting essentially of, as a major part, 85.0 to 99.95 mole percent of zinc oXide and, as an additive, 0.05 to 15.0 mole percent of at least one member selected from the group consisting of manganese fluoride, `magnesium fluoride, calcium fluoride, cadmium fluoride, potassium fluoride, chromium fluoride, sodium fluoride, cobalt fluoride, copper 3 fluoride, ferric fluoride, lanthanum fluoride and lithium fluoride. Such a voltage variable resistor has non-ohmic resistance due to the bulk itself. Therefore, its C-value can be changed without impairing the n-Value by changing the distance between said opposite surfaces. The shorter distance results in the lower C-value.

The negative resistance shown in FIG. 2 can be obtained when said sintered body consists essentially of the batch following composition listed in Table 1.

The larger -value can be obtained when said additve consists essentially of at least two members selected from the group conssting of 0.1 to 7.0 mole percent of manganese fluoride, 0.1 to 3.0 mole percent of copper fluoride and 1.0 to 5.0 mole percent of cobalt fiuoride.

According to the present invention, the negative resistance can be improved in the stability for ambient temperature and the electric load life test when said additive consists essentially of 0.1 to 3.0 mole percent of manganese fluoride and 0.5 to 5.0 mole percent of magnesum oxide.

The -value is elevated and at the same time the stability for ambient temperature and the electric load life test is improved when said additive consists essentially of 0.1 to 2.0 mole percent of manganese fluoride, 0.1 to 1.0 mole percent of lanthanum oxide and 0.5 to 5.0 mole percent of magnesium oxide.

According to the present invention, the -value is further elevated and the stability is remarkably improved when said additive consists essentially of 0.1 to 2.0 mole percent of manganese fluoride, 0.1 to 2.0 mole percent of cobalt oxide, 0.1 to 1.0 mole percent of lanthanum oxide and 0.5 to 5.0 mole percent of magnesium oxide.

The sintered body 1 can be prepared by a per se well known ceramic technique. The starting materials of the batch compositions described in the foregoing description are mixed in a wet mill so as to produce homogeneous mixtures. T he mixture is dried and pressed in a mold into desired shapes at a pressure from 100 kgJcm. to 1000 kg./cm. The pressed bodies are sintered in air at a given temperature for 1 to 3 hours, and then furnace-cooled to room temperature (about to about C.).

The available sintering temperature is determined in view of electrical resistivity, non-linearity and stability and ranges from 1000 to 1450 C.

The mixtures can be preliminarily calcined at about 700 C. and pulverized for easy fabrication in the subsequent pressing step. The mixture to be pressed can be admixed with a suitable binder such as water, polyvinyl alcohol, etc. These methods are described as an example of fabrication method and should not be construed as limitative.

The sintered bodies are provided, at the opposite surfaces thereof, with electrodes in any available and suitable method such as electroplating method, vacuum evaporation method, metallizing method by spraying or silver painting method.

The non-ohmic resistances are not practically aifected by the kinds of electrodes used, but are afected by the thickness of the sintered bodies. Particularly, the C-value, V value and V value vary in proportion to the thickness of the sintered bodies, while the n-value and -value are substantally independent of the thickness.

Lead wires can be attached to the electrodes in a per se conventional manner by using conventional solder having a low meltng point. It is convenient to employ a conductive adhesive comprising silver powder and resin in an organic solvent in order to connect the lead wires to the electrodes.

Voltage variable resistors 'according to this invention have a high stability in temperature and in the load life test, which is carried out at 70 C. at a rating power for 500 hours. The r value, C-value, V -value and -value do not change remarkably after heating cycles and load life test. It is advantageous for achievement of a high stability in humidty that the resultant voltage variable resistors are embedded in a humidty proof resin such as epoxy resin and phenol resin in a per se well known manner.

Presently preferred illustrative embodiments of the invention are as follows.

EXAMPLE 1 A mixture of Zinc oxide and additives in a composition of Table 2 are mixed in a wet mill for 3 hours. In Table 2, the remainder consists of zinc oxide. The mxture is dried and then calcined at 700 C. for l hour. The calcined mixture is pulverized by the motor-driven ceramic mortar for 30 minutes and then pressed in a mold into a shape of 17.5 mm. in diameter and 2.0 mm. in thickness at a pressure of 500 kg./cm.

The pressed body is sintered in air at 1l50 C. for l hour, and then furnace-cooled to room temperature (about l5 to about 30 C.). The sintered disc is lapped at the opposite surfaces thereof by silicon carbide in a particle size of 600 meshes. Resulting sintered disc has a size of 16 mm. in diameter and 1.5 mm. in thickness. The silver paint electrodes commercally available are attached to the opposite surfaces of sintered disc by painting. Then lead wires are attached to the silver electrodes by soldering. The electric characteristics of the resultant resistors are shown in Table 2. It will be readily understood that the Zinc oxide sintered body incorporated with additives listed in Table 2 have -an excellent voltage-nonlinear property and particularly a given amount of additives result in so-called negative resistance property.

EXAMPLE 2 Zinc oxide and additives listed in Table 3 are mixed, dried, calcined and pulverized in the same manner as those of Example 1. The pulverized mixture is pressed in a mold into a disc of 17.5 mm. in diameter and 5 mm. in thickness at a pressure of 500 lag/cm?.

The pressed body is sintered in air at 1350 C. for 1 hour, nd then furnace-cooled to room temperature. The sintered disc is ground at the opposite surfaces thereof into the thickness shown in Table 3 by silicon carbide in a particle size of 600 meshes. The ground disc is provided with the electrodes and lead wires at the opposite surface in a manner similar to that of Example l. The electric characteristcs of the resultant resistors are shown in Table 3; the V -value and the V -value the C-value, vary approximately in proportion to the thickness of the sintered disc while the -value, the I -value, and the -value are essentially independent of the thickness. It will be readily realized that the voltage nonlinear properties of the resistors are attributed to the sintered body itself.

EXAMPLE 3 Zinc oxide incorporated with additives in the composition of Table 4 is fabricated into the voltage-variable resistors by the same process as that of Example 1. The electric characteristics of the resultant resistors are shown in Table 4. It can be easily realized that the additive of at least one member selected from the group consisting of 0.1 to 7.0 mole percent of manganese fluoride, 0.1 to 3.0 mole percent of copper fluoride and 1.0 to 5.0 mole percent of cobalt fluoride results in the larger value of EXAMPLE 4 Zinc oxide incorporated with additives in the composition of Table 5 is fabricated into the voltage variable resistors by the same process as that of Example 1. The resulting resistors are tested according to the methods used in the electronic Component parts. The load life test is carried out at 70 C. ambient temperature at 0.5 watt rating power for 500 hours. The heating cycle test is carried out by repeating 5 times by cycle in which said resistors are kept at C. ambienttemperature for 30 minutes, cooled rapidly to -20 C. and then kept at such temperature for 30 minutes. Table 5 shows a difference in V VaIUeS, V -values and -values after the load life test. It can be readly realized that the combination of manganese fluoride and magnesum oxide as additve is effective for the electrcal and environmental 6 Table 1 Zinc oxide (mole percent): Additve (mole percent) Stablt 933-999 Manganese fluoride 0.1-7.0. EXAMPLE 5 97.0-99.7 Magnesiun fluorde 0.3 3.o.

5 95.0-99.5 Calcium fiuoride 0.5-5.0. Zinc oxide incorporated with additives of Table 6 is 99.0-99.9 Cadmun fluoride 0.1-1.0. fabricated into the voltage variable resistors by the same 98,0-999 Potassum fluoride 0.1-2.0. process as that of Example 1. The electrical properties 98.0-99.9 Chromum fiuorde 0.1-2.0. of the resulting resistors are shown in Table 6. It will be 92.0999 Sodium fluoride 0.1-8.0. readily seen that the combinaton of manganese fluoride, 10 9S.O -9 9.0 Cobalt fluo'de 1.0-5.0. lanthanum oxide and magnesum oxde, particularly 97.0-999 Copper fluoride 0.1-3.0. when cobalt oxide s added to said combination, as addi- 98.0-99.9 Ferric fiuoride 0.1-2.0. tives is excellent in negative resistance property and in 98.5-99.7 Lanthanum fluoride 0.3-1.5. the electrical and environmental stability. 9'8.0-99.2 Lithum fluoride 0.8-2.0.

TABLE 2 Amount,

Mold C (at 1 Additive percent ma.) n V(v) Iflma.) V (v.)

Mangaese fluoride o o Mangesium fluoride %og Calcium fluoride 0. 05 0.1 0.5 1.0 2.0 5.0 10.0 15.0

Cadium fluoride 0. 05 v 0.1 0.5 1.0 2.0 5.0 10.0 15.0

Potassium fluor-ide 0. 05

Chromum fluoride Sodium fiuoride Cohalt fiuoride 0605 Copper fiuoride 06 0 1. 3. 10. 15.

Ferr-ie fluoride TABLE 6 Additive, mole percent Magnesum Lanthamm Magnesium Cobalt fluoride ocde 39437716911 91097440706.08687859887968747565196364778182 6 5 5 &ai&iL ku&4 A &A AmAmArma&AmA 4 4 &ama/2222222222222222222222222&

161150915187355369313593347944766693675864566944657596 4 3 A 3 2 lam&&RWLZZ2322333&232%&ZLLLLLLLLLLIILLLLLLLLLLLLLLQ 00155500011155000111555000 111000111000111000111000555 LLQ uauLLL&QOQQLLLQ&QQQQLLLQQOQLLLQQQLLLQQQLLLQflQLLLQQQ 1001001001001010010010010001111110000 0000 00 0 LZQLZQLZ&L202QLZQLZQLZQLZLQQQQQQZZZZZZQQQGQQZZZZZZLLL What is claimed is: 3.0 mole percent of copper fluoride and 1.0 to 5.0 mole 1. A composition adapted for a voltage variable resistor percent of cobalt fiuoride. consistng essentially of, as a major part, zinc oxide and, 4. A composition adapted for a voltage variable reas an additive, 0.05 to 15.0 mole percent of at least one sistor in claim 1, wherein said additive consists essentially member selected from the group consisting of manganese of 0.1 to 3.0 mole percent of manganese fiuoride and 0.5 fluoride, magnesium fluoride, calcum fluoride, cadmium to 5.0 mole percent of magnesum oxide. fluoride, potassium fluoride, chromium fluoride, sodium 5. A composition adapted for a voltage variable refluoride, cobalt fluoride, copper fluoride, ferric fluoride, sistor in claim 1, wherein said additive consists essentially lanthanum uoride and lthium fluoride. of 0.1 to 2.0 mole percent of manganese fluoride, 0.1 to 1.0 mole percent of lanthanum oxide and 0.5 to 5.0 mole percent of magnesium oxde.

2. A composition adapted for a voltage variable resistor in claim 1, wherein said additive consists essentially 6. A composition a sistor in c of one member selected from the group consisting of 0.1 to 7.0 mole percent of manganese fluoride, 0.3 to 3.0 mole pel-cent of magnesium fluoride, 0.5 to 5.0 mole percent of calcium fluoride, 0.1 to 1.0 mole percent of cadmium fluoride, 0.1 to 2.0 mole percent of potassium fluoride, 0.1 to 2.0 mole percent of chromium fluoride, 0.1 to 8.0 mole percent of sodium fluoride, 1.0 to 5.0 mole percent of cobalt fluoride, 0.1 to 3.0 mole percent of copper fluoride, 0.1 to 2.0 mole percent of ferric fluoride,

.3 to 1.5 mole percent of lanthanum fluoride and 0.8 to 2.0 mole percent of lithium fiuoride.

ble DOUGLASJ.DRUMMOND,Primary Examner U.S. Cl. X.R. 252-521 3. A composition adapted for a voltage vara sistor in claim 1, wherein said addtve consists essentially of at least two members selected from the group consisting of 0.1 to 7.0 mole percent manganese fluoride, 0.1 to 

